While custom variables are great – you only get 5 of them, or so Google says. There is an undocumented feature that basically gives you as many custom variables as you want. It’s as simple as setting the number of maximum custom variables as you want to use. Here is the Google Analytics function to use:

[important update July 2, 2010]Omar brought to my attention that when drilling down via the standard GA interface, when you pick a custom variable after the 5th one (in order that is was set, not in the order on the interface) you get an error saying:

An Error Has Been Detected

Please try again. Thank you for your patience.

My gut tells me that this is ONLY an issue with the interface and that internally the data is still being recorded, but I can’t know for sure.
In any case it seems that from a functional standpoint creating more than 5 custom variables is fairly limited since currently you can only see the variable names and not the values for slots 6 and up.